'89 Maxima No Start

So I'm working on an '89 Maxima that originally came in with a battery
drain problem. Found that the "main relay" had been grounded
constantly on (and was keeping the ECU powered up constantly), so I
disconnected the jumper-ground and that took care of the battery drain.

Without the relay grounded, however, the car will crank but not fire.
The wiring is such that the ECU provides a ground signal out to the
"main relay" which, when energized, switches power to (among other
things) the MAF, CKP, and other sectors of the ECU (specifically
controls for fuel pumps and injectors). I've checked continuity on the
ground signal wire from the ECU to the "main relay", and it is fine. A
jumped ground to this wire at the ECU connector will energize the
relay, and the car will start and run fine.

Have spent quite a few hours double-checking all inputs (powers,
grounds, etc.) to the ECU, and they all appear to be fine. Finally,
ordered an ECU for the car. The new ECU doesn't work either (same
problem). It is a used, but guaranteed good part.

Sooo... Anybody familiar with Nissans know of any system or ECU input
that might control this ground signal? Or should I be worrying about
exchanging the supposedly good ECU for another?

One last note: the new ECU is Fed-spec, while the old one was Ca-spec.
All info I have found seems to indicate no problem with the
interchange, and the Ca-spec. computers are just not available (hence
the purchase of a Fed-spec. box). Might there be some unknown factor
here causing the problem?

Thanks for any help,
Patrick